This invention relates to silicon photodiodes. More particularly, it relates to silicon photodiodes that are compatible with standard CMOS VLSI processing techniques that are effective in the visible to near infra-red spectral range.
Si technology dominates the microelectronics marketplace. Optoelectronics, on the other hand, has been largely based on III-V semiconductors such as GaAs and InP and additional ternary and quaternary compounds grown on these materials. This is largely because of the stronger interaction (absorption and emission) with electromagnetic fields at above-bandgap energies in these direct bandgap semiconductors as compared with the weaker absorption of indirect bandgap Si over much of the near infrared and visible spectral region. Thus, III-V semiconductor lasers have been developed over a wide range of wavelengths from the red at 630 nm to the near infrared at 2 .mu.m or even longer wavelengths. Lasers fabricated directly from Si have not yet been achieved.
The weaker absorption of Si also impacts the design and performance of optical detectors. Over much of the visible and near-IR, Si absorption lengths are on the order of 10's of micrometers. This dictates a vertical structure for Si detectors with a p-doped upper layer, a thick intrinsic layer that provides the absorption and an n-doped substrate. The roles of the n- and p- layers could also be reversed. Alternatively, low doping levels resulting in a wide depletion region for the p-n junction may be used. Two difficulties with this detector configuration are 1) the detectors are relatively slow because of the drift times across the wide intrinsic region and 2) fabrication requires deep diffusions and/or thick growths and the detectors are difficult to integrate with VLSI circuits whose active device layers are confined to very narrow regions (.about.1-2 .mu.m or less) at the top of the wafer. Certainly, the necessity of accessing the substrate region in these Si detectors makes them incompatible with standard VLSI device fabrication.
Optical interconnections in high-performance computer systems are a potentially important application of optoelectronics. Traditional electrical (wire) based interconnections are approaching speed and density limitations imposed by capacitance, crosstalk, and thermal effects. Optical signals propagate at the speed-of-light, are inherently more immune to crosstalk, and potentially require lower powers. Many possible architectures are under investigation. One aspect is common, however, eventually the optical interconnect must be integrated with the Si circuitry of the computer. Most approaches today are investigating hybrid techniques in which a III-V detector is somehow coupled to the Si circuit. This requires complex and highly precise and delicate packaging and assembly and will negatively affect yield in a production environment.
Even the operating wavelength of an optical interconnection system is far from clear at this point. Almost certainly, semiconductor-diode-based optical sources will be used; inevitably, this will be true for any volume application as the optical interconnect technology penetrates beyond the very high-end systems. As mentioned above, this constrains the wavelength to the range 0.6-2 .mu.m. Within this range several wavelengths offer significant advantages: .about.860 nm (wavelength range of GaAs/GaAlAs lasers, among the best developed and highest power sources), and 1.3 and 1.55 .mu.m (wavelengths of long distance fiber communication systems).
There are also advantages in terms of diode laser lifetime in working at a number of other wavelengths such as 800 nm or 980 nm.
This background points to the need for improved Si detector performance, fully compatible with VLSI technology.